powerpc: Drop old workaround for old gcc

Very old versions of gcc defined _BIG_ENDIAN and _LITTLE_ENDIAN. So to
work around that, we undefined them here. However, that causes problems
for programs that do:

(and many other variations on that theme). Since this often is the
result of weirdly nested includes in the ports world that are hard to
unwind, drop this workaround to help more ports build out of the box.
If there's still an issue here (and my testing hasn't shown it), we'll
fix the issue in a brand-new way once I have a reproducer.

This fixes the mesa-devel build, and others

Sponsored by: Netflix
Tested by: pkubaj
MFC After: 3 days
Differential Revision: https://reviews.freebsd.org/D38564

(cherry picked from commit e35448a901aa56aa232efbc4fe7a73a9bdac3511)